Automatic frequency control



OctJB, 1940. N. M. RUST 2,217,294

AUTOMATIC FREQUENCY CONTROL Filed Nov. 30. 1937 cow/m cow/em M CONTROL VOLTAGE VOLTAGE ERROR/N TUNING ERROR IN TUNING ERROR IN TUNING CONTROL VOL T5 INVENTOK NOEL M. RUST ATTORNEY.

Patented Oct. 8, 1940 HTED STATES PATENT OFFICE Noel Meyer Rust, Chelmsford, England, assignor to Radio Gorporation of America, a, corporation of Delaware Application November 30, 1937, Serial No.l77,191 In Great Britain December 31, 1936 3 Claims.

This invention relates to radio and like receivers, and more particularly to automatic frequency control systems for use therein.

By the expression automatic frequency con- 5 trol system (AFC. system) as herein employed is meant a system whereby the tuning of a receiver as a whole is automatically maintained correct for a station to be received when once the said tuning has been setusually manuallywithin a predetermined range on either side of the correct tuning position. AFC. systems are sometimes (rather loosely) termed automatic tuning systems, since a measure of automatic tuning is obtained thereby, although, in fact, the

AFC. system provides merely accuracy of tuning, selection as between stations bein effected by other means, usually an ordinary manually controlled tuning knob.

For the purposes of the present specification,

AFC. systems may be regarded as broadly divisible into two types (1) that in which a symmetrical AFC. control action is obtained, and (2) that in which an asymmetrical A.F.C. control action is obtained. By a symmetrical action AFC. system is meant a system such that a departure from accurate tuning of a certain amount on one side of the correct tuning position will produce a tuning correction which is equal and opposite to that produced as the result or a departure from accurate tuning of the same amount on the other side of the correct tuning position.

The present invention is applicable to A.F.C. systems whether of the symmetrical action type or of the asymmetrical action type, although it is particularly advantageous when applied to the latter type. The object of the invention is to effect what may be termed sharpening of the peaks of the operating characteristic of AFC.

systems.

In the drawing:

Fig. 1 shows the operating characteristic of a symmetrical AFC. system,

Fig. 2 is the characteristic of an asymmetrical AFC. system,

Fig. 3 illustrates the characteristic of a diferent type of asymmetrical AFC. system,

Fig. 4 shows an arrangement embodying the invention,

Fig. 4a illustrates the characteristic of the discriminator of Fig. 4,

Fig. 5 shows a modified arrangement,

Fig. 5a illustrates the discriminator characteristic of Fig. 5,

Figs. 6 and 7 are mutually perpendicular schematic views of a double coil coupling unit used in Figs. 4 and 5.

The accompanying Figure 1 shows a typical operating characteristic for a symmetrical action AFC. system. This curve connects tuning cor- 5" recting voltage (ordinates) applied by the A.F.C. system to a control portion of the circuit to secure accuracy of tuning, with error in tuning (abscissae); error in tuning below the correct tuning position being regarded as negative, and 10 error in tuning above the correct tuning position being regarded as positive. As will be seen this curve commences to rise from the abscissa line at a point on thev negative side of the origin, reaches a maximum above the abscissa line at a 15 point nearer the ordinate line but still on the negative side thereof, descends fairly sharply to the origin, then rises fairly sharply to a maximum below the abscissa line and on the positive side of the ordinate line, and then falls away to the 2 abscissa line, the half of the curve below the abscissa line being symmetrical with respect to the half of the curve above it. The origin is the point of correct tuning.

The accompanying Figure 2 shows in similar 51-: manner the operating characteristic for one general type of asymmetrical action A.F.C. system. Here the characteristic curve first rises in the same way as the symmetrical curve of Figure 1 to a maximum on the negative side of the ordinate line and above the abscissa line, and then falls away to a point on the ordinate line near the origin. The curve, however, instead of proceeding below the abscissa line commences sharply to rise again above the abscissa line and reaches a second maximum; the half of the curve on the positive side of the ordinate line being substantially a mirror image of the part of the curve on the negative side. of the ordinate line.

The accompanying Figure 3 shows the operating characteristic of yet another general type of asymmetrical A.F.C. system. Here the operating characteristic has a portion like that half of the characteristic of Figure 1 which is above the abscissa line and to the negative side of the ordinate line, but, after descending substantially to the origin, the curve proceeds below the abscissa line on the positive side of the ordinate line,but the portion of the curve to the right of the ordinate line is of much smaller amplitude 50 than the portion to the left of it, the curve thus being asymmetrical with respect to all the axes.

In all three of the typical cases just mentioned the curves have maxima or. peaks which are important to the; operation; In*thercase=of a .52

point of correct tuning should be'near the origin,

symmetrical action system with a curve as shown in Figure 1, it is desirable to obtain maximum sharpness in the region between the two peaks, one of which is above the abscissa line and the other below it. In the second and third typical curves above described, it is desirable to obtain maximum sharpness between that peak which is to one side of the ordinate line (in the usual case to the negativeside thereof) and the origin. To put the matter in another way, in all three cases the idea operating characteristic curves are of saw tooth form, the side of the tooth of lesser slope defining the region where AFC. is taking effect, while the steep side defines the accuracy with which the control is effected. The steep side should, therefore, be as nearly vertical as possible. In the asymmetrical case the characteristic is represented by a single saw tooth while in the symmetrical case (the first case herein mentioned) the characteristic is given by a double saw tooth the second tooth being inverted, the steep sides of the two teeth joining one another. In the symmetrical case the width of saw tooth base defines the region of control or pull infregion. In the asymmetrical case the or some way up the steeply sloping side of the saw tooth.

In all automatic frequency control systems the tuning, correcting, or control voltages are obtained'by means including suitable discriminating networks, and the main object of the, present invention may accordingly be re-stated as consisting in the provision, for such networks, of

filters of such nature that the required sharp operating characteristics are reliably and easily obtained.

According to this invention a required sharpness in the operating characteristic of an automatic frequency control system is obtained by "including in the discriminating network therefor,

at least one filter of the tuned circuit type having two coils mutually coupled so that the mutual inductance constitutes in effectpart of a shunt limb in the filter as a whole, said shunt limb also including a condenser which tunes with the mutual inductance. i

The present invention though not limited to its application thereto, is well suited to application to superheterodyne receivers having auto- 'matic frequency control systems actuated by energy derived from an intermediate frequency stage and the said invention will be particularly described as applied to such receivers.

Referring to Figure 4, which shows diagrammatically one way of carrying out this invention, an intermediate frequency valve IFV of a superheterodyne receiver has in its anode circuit a coil I which is coupled to a secondary coil 2 which acts as the input coil for a filter provided in accordance with this invention.v The said input coil inductance 2 is connected in a loop circuit which contains in series, in the order stated, the said coil 2, two fixed condensers 3, 4, and two inductances 5, 6. A third fixed condenser l is connected between the junction point of the two I the third condenser 7 are of such size as together to be series resonant to the said frequency. The resultant characteristic curve produced will then be of the asymmetrical type rising to a maximum in the desired manner and then falling away very sharply indeed. This curve is represented to the right in Figure 4a. Of course, any means known per se for utilizing the voltage output from the discriminating filter for A. F. C. control may be substituted for the diode and output resistance arrangement above described.

In the modification shown in Figure 5, two coils la lb in series are included in the anode circuit of the intermediate frequency amplifier valve IFV, and each of these coils is coupled to the input coil 2a or 2b of a filter like that shown in Figure 4. In one filter of Figure 5 parts corresponding to those of the filter of Fig. 4 are marked with the same references, but with the suffix a attached while in the other filter the sufilx b is added. The two filters, though similar, are dimensioned slightly differently, one (the upper one in Fig. 5) being dimensioned to have its resonancies slightly above the predetermined intermediate frequency, and the other (the lower one) being dimensioned to have its resonancies an equal amount slightly below the predetermined intermediate frequency. A diode Ha (or Hb) in series with a capacity shunted resistance combination 8a 9a (or 81) 9b) is provided for each filter,the series combination of diode and capacity shunted resistance being connected in each case between the junction point between the two inductances 5a 6a (or 519 6b) in the appropriate filter, and the junction point between the inductance 5a (or 517) and the condenser la (or 41)).

In each case the anode 1201. (or [217) of the diode is connected to-the last mentioned junction point. The ends of the load resistances remote from the diode cathodes llla (10b) are connected together and to the earthed cathode point of the intermediate frequency amplifier valve IFV from which the double filter is fed. Control voltage for A. F. C'. control is taken off at l3, l4,,from between the two diode cathodes Illa, 101). This embodiment provides a symmetrical action for the response curves of the filters combined to produce a symmetrical over-all or combined char-. acteristic curve (as shown to the right in Fig. 5a) which has a very sharp descending portion following the ordinate line very closely between the tWo peaks. By suitably choosing the stagger of the two filters withrespect to the predetermined intermediate frequency a very good result indeed may be obtained.

In any of the various possible embodiments of the invention the two filter inductances 5, 6, 5a, 6a, or 512, 6b which are coupled may be and preferably are embodied in a double coil coupling unit. A preferred form of such unit is shown in mutually perpendicular schematic views in Figs. 6 and 7. Here the two coils are mounted upon a rectangular core RC built up of interleaved laminationsfor example L shaped lami- Y nations-of suitable high frequency ferro-magnetic material such as the material known under the name Ferromould. The coils occupy two opposite limbs of the core, each coil having one half on one limb and the other on the opposite limb. Preferably the coils are sectionalized; for example, as indicated in Fig. '7 each may be wound in eight pancake sections on eight bobbins, four of which will be on one limb and four on the opposite. An electrostatic screen ES of sheet copper extends across the core andseparates the two coils, said screen having two windowsor, in other words three parallel limbs at right angles to and joining two further parallel limbs. The central one of the three limbs passes down the single window in the core and the outer two of the said three limbs pass each outside the wound limbs of the core, one on each side. The screen is, of course, suitably slotted or cut as at SES to prevent its acting as a short circuited turn. A unit as above described can readily be constructed to give quite tight magnetic coupling (say about 40%) without appreciable electrostatic coupling.

Having now particularly described and ascertained the nature of said invention, and in what manner the same is to be performed I declare that what I claim is:

1. In the discriminator network of an automatic frequency control system for a superheterodyne receiver, a filter which includes in series in a loop circuit in the order stated two condensers and two mutually coupled inductances, a third condenser connected between the junction point of said inductances and the junction point of said condensers, the filter as a whole being resonant at the frequency which should appear therein when correct tuning is accomplished, and the mutual inductance between said inductances being series resonant with the third condenser at said frequency, means for impressing intermediate frequency energy of the receiver upon said filter, means for deriving a direct current control voltage from the energy transmitted through the filter, and the filter having a response characteristic such that maximum control voltage is produced at a frequency which is only slightly spaced from the filter frequency.

2. A discriminator network which includes two filters each including in series in a loop circuit in the order stated two condensers and. two mutually coupled inductances, a third condenser connected between the junction point of said inductances and the junction point of said condensers, one filter being resonant as a whole at a frequency a little'abo-ve the frequency which should appear therein when correct tuning is accomplished, the mutual inductance between the inductances of this filter being series resonant with the third condenser thereof at saidfirst mentioned frequency, the other filter being resonant as a whole at a second frequency an equal amount below the frequency which should appear when correct tuning is accomplished, the mutual inductance. between the inductances' of the said other filter being series resonant with the third condenser thereof at the frequency at which the filter as a whole is resonant, means impressing signal energy upon both of said filters, means rectifying the signal enegy tansmitted by said filters, and the network as a whole having a response characteristic such that maximum rectified voltage is produced at frequencies slightly spaced on either side of the means of said filter frequencies.

3. In a superheterodyne receiver of the type comprising a source of intermediate frequency energy and a frequency discriminator network for automatic frequency control; the improvement which is characterized by the discriminator network including at least one filter circuit which comprises a coil coupled to said source, said coil being connected in a loop circuit consisting in series, in the order stated, of said coil, a pair of condensers and a pair of inductances, a third condenser being connected between the junction of said pair of condensers and the junction of said inductances, said inductances being coupled in series opposition, said filter as a whole being resonant substantially to said intermediate frequency, the mutual inductance between said inductances resonating said third condenser to said filter frequency, means for rectifying the energy transmitted through the filter, and the filter having a characteristic such that substantially no rectified output voltage is produced at the filter frequency, while maximum rectified voltage is produced at a frequency slightly separated therefrom.

NOEL MEYER RUST. 

